Light olefins, defined herein as ethylene, propylene, butylene and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, light olefins are produced by cracking petroleum feeds. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies based on alternative feeds have increased.
An important type of alternate feed for the production of light olefins is oxygenate, such as, for example, alcohols, particularly methanol and ethanol, dimethyl ether (DME), methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
The conversion of oxygenates to olefins generates by-products whose presence is undesirable for subsequent applications of the collected olefins. Methanol conversion can be carried out over small pore molecular sieves having a chabazite structure and, more specifically, silicoaluminophosphates such as SAPO-34. These small pore molecular sieves are very efficient in converting methanol to light olefins, primarily ethylene and propylene. However, as a by-product of the methanol conversion chemistry, small amounts of oxygenates are produced. Typically, aldehydes and/or ketones, as well as ethers can be present as by-products. The presence of C2 to C5 aldehydes and C3 to C6 ketones can lead to formation of undesired compounds such as red oils, which create problems during subsequent processing. Accordingly, these oxygenates need to be removed from the olefinic product streams to meet product quality requirements. Even though these oxygenates may be present in only small quantities, a significant investment of capital is needed for their removal by conventional separation technology, e.g. distillation, owing to their nearness in boiling point to the boiling points of desired olefin products. Accordingly, it would be desirable to provide a means for eliminating at least some of these oxygenates by reacting them over a suitable catalyst with a reactant to produce less troublesome components, e.g., higher boiling components.
Methods for recovering and recycling dimethyl ether from a methanol-to-chemical conversion reaction using a dimethyl ether absorber tower are disclosed in U.S. Pat. No. 4,587,373 to Hsia.
Stud. Surf. Sci. Catal. (1985), 20 (Catl. Acids Bases), 391–8, discusses low temperature conversion of dimethyl ether over Pt/H-ZSM-5 in the presence of hydrogen by a bifunctional catalyzed reaction.
Stud. Surf. Sci. Catal. (1993), 77 discusses hydrogenation of oxygenates such as dimethyl ether over a Ni/Al2O3 catalyst to form methane.
U.S. Pat. No. 5,491,273 to Chang et al. discloses conversion of lower aliphatic alcohols and corresponding ethers to linear olefins over large crystal zeolites, e.g., ZSM-35 containing a hydrogenation component of Group VIA and Group VIIIA metals.
DE3210756 discloses a process for converting methanol and/or dimethyl ether feed to olefins by reacting the feed over a pentasil type zeolite catalyst, separating C2–C4 olefins, methane and water from the reaction product and catalytically hydrogenating the remaining components over Co—Mo supported on alumina, optionally preceded by hydrogenation over a Group 8 noble metal for polyunsaturated, non-aromatic compounds.
U.S. Pat. No. 4,912,281 to Wu discloses converting methanol or methyl ether to light olefins in the presence of hydrogen and ZSM-45 which is highly selective to C2–C4 olefins, especially ethylene.
DE2720749 discloses converting lower aliphatic ethers to hydrocarbons in the presence of amorphous, non-acid-activated Al silicate.
U.S. Pat. No. 4,625,050 to Current discloses the use of carbonylation to convert dimethyl ether to methyl acetate and ethanol (as well as minor amounts of methyl formate and propanol) over hydrogen and CO in the presence of heterogeneous NiMo catalyst on an alumina support.
EP-229994 discloses the removal of dimethyl ether as an impurity (1–500 wppm) of olefinic hydrocarbon feedstock by passing the feedstock through an adsorbent mass of crystalline zeolite molecular sieve having the crystal structure of faujasite at 0–60° C. and 0.15–500 psia to selectively absorb dimethyl ether.
“Bifunctional Condensation Reactions of Alcohols on Basic Oxides Modified by Copper and Potassium”, M. J. L. Gines and E. Iglesia, J. Catal., 176, 155–172 (1998) discloses alcohol dehydrogenation and condensation reactions involved in chain growth pathways on Cu/MgCeOx which lead to formation of isobutanol with high selectivity via reactions of higher alcohols with methanol-derived C1 species in reaction steps.
Given the difficulties presented in removing oxygenate by-products of oxygenates to olefins processes such as aldehydes and/or ketones, as well as ethers, it would be advantageous to remove at least one or more of these by-products with techniques that do not require dedicated equipment for superfractionation, water washing, etc.